1. Field of the Invention
The present invention relates to a semiconductor device having a nonvolatile semiconductor memory element capable of writing, reading, and erasing.
Note that a “semiconductor device” in this specification refers to a general device that can function using semiconductor characteristics, and electro-optical devices, semiconductor circuits, and electronic devices can all be considered semiconductor devices.
2. Description of the Related Art
A nonvolatile semiconductor memory element is a semiconductor element capable of electrically rewriting and storing data even when a power supply is turned off. As the nonvolatile semiconductor memory element, nonvolatile memory transistors having a structure similar to that of a metal oxide semiconductor field effect transistor (MOSFET) are classified into two major groups in terms of charge storage means. One is a floating gate (FG) type in which a charge storage unit is formed of a conductive layer between a channel formation region and a gate electrode; the other is a metal-oxide-nitride-oxide-silicon (MONOS) type and a metal-nitride-oxide-silicon (MNOS) type each of which uses a charge trapping layer as a charge storage means.
In many of the MONOS memory transistors and the MNOS memory transistors, a silicon nitride film that contains many charge traps is used as a charge storage means. In order to improve charge retention characteristics of the MONOS memory transistors and the MNOS memory transistors, silicon nitride films have been researched.
For example, improvement in retention characteristics of a memory transistor by provision of a silicon nitride film having a two-layer structure including different compositions or composition ratios is described in References 1 to 4. Improvement in retention characteristics by provision of a silicon nitride film having a three-layer structure including different composition ratios is formed is described in Reference 5.
In Reference 1 (Japanese Examined Patent Application Publication No. H2-59632), a silicon nitride film containing Si—H bonds is formed as a lower layer, and a silicon nitride film which hardly contains Si—H bonds is formed as an upper layer. A silicon nitride film having such a two-layer structure is formed by a CVD method in which SiH4 and NH3 are used as source materials, providing that a formation temperature when a silicon nitride film is formed as a lower layer is set at 700° C. to 900° C. and a formation temperature when a silicon nitride film is formed as an upper layer is set at 900° C. or higher.
In Reference 2 (Japanese Examined Patent Application Publication No. S59-24547), a silicon nitride film which contains a lot of Si is formed as a lower layer, and a silicon nitride film which contains a lot of N is formed as an upper layer. In order to form a silicon nitride film having such a two-layer structure, a CVD method in which SiH4 and NH3 are used as source materials is used, and a flow ratio of NH3/SiH4 is set at 50 to 150 when a lower layer is formed and the flow ratio of NH3/SiH4 is set at over 300 when an upper layer is formed.
In Reference 3 (Japanese Published Patent Application No. S63-205965), a silicon nitride film having a relatively high conductivity is formed as a lower layer, and a silicon nitride film having a relatively low conductivity is formed as an upper layer by a CVD method. As a condition to form a silicon nitride film having such a two-layer structure, the following is described: heating temperature is at 700° C. to 800° C., SiH2Cl2 and NH3 are used as source materials, a flow ratio of NH3/SiH2Cl2 is set at 0.1 to 150 when a lower layer is formed, and the flow ratio of NH3/SiH2Cl2 is set at 10 to 1000 when an upper layer is formed.
In Reference 4 (Japanese Published Patent Application No. 2002-203917), a silicon nitride film having a two-layer structure in which the charge trap density of an upper layer is set higher than that of a lower layer is formed by a CVD method. To form a silicon nitride film having such a two-layer structure, the following is described: SiH4, SiH2Cl2, or the like in which the composition ratio of chlorine is lower than that of a silicon source gas used when a lower layer is formed is used as a silicon source gas used when an upper layer is formed. In Reference 4, by changing the composition ratio of chlorine of a silicon source gas, a silicon nitride film containing a larger number of Si—Cl bonds than Si—H bonds is formed as a lower layer and a silicon nitride film containing a lot of Si—H bonds is formed as an upper layer.
In Reference 5 (Japanese Published Patent Application No. H3-9571), a silicon nitride film having a three-layer structure is described, in which charge trap level density of a second-layer silicon nitride film is higher than those of the other layers and the concentration of Si of the second-layer silicon nitride film is increased. To form a silicon nitride film having such a three-layer structure, the flow rate of SiH2Cl2 is increased at the time when the second layer is formed.